Gears are used for increasing the torque of the source of rotary motion having high angular momentum and low torque. This high torque is necessary for performance of work. This phenomenon of increase in torque is called gear reduction and is brought about by coupling of a smaller gear called the pinion with a larger gear. This results in reduction of torque at the expense of angular momentum. Such a gearbox is called a reducer. One more application of gears is to change the axis or plane of rotary motion with or without gear reduction.
When you open a gearbox you will see that the inner construction is very simple. Inside you will find two gears coupled with one another. The gears may be of spur, helical, cycloid, worm or bevel type. In case of gear reduction, the diameter of the output is larger than that of the input gear. If only a change in direction is required, the size of the gears is the same. Spur gears are used for heavy load but are noisy. If the load is comparatively lesser, helical gears are preferred as they are silent in operation due to gradual engagement. If change of plane of rotation is required, hypoid gears are used.
The gear may be either of metal or plastic. This entire arrangement is enclosed in metallic or plastic housing. The point of contact of the gear teeth is well lubricated with gear oil. The gear oil must be very clean and free of abrasive materials to avoid wearing of the gears.
A gearbox is used in turbines, windmills, grinders, etc. to change the direction of the rotary motion. In automobiles a gear box is used for transfer of to power of the engine to the wheels through a differential.